Low Current Thyristor-Based Dimming

ABSTRACT

An apparatus for low current thyristor-based dimming includes a thyristor, a secondary load switchably connected to an output of the thyristor, and a sensor connected to the secondary load. The sensor is adapted to connect the secondary load to the thyristor when the output of the thyristor falls below a predetermined level.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 61/168,576 entitled “Low Current TRIAC-Based Dimming”,filed Apr. 11, 2009, and to U.S. Provisional Patent Application No.61/182,680 entitled “Low Current TRIAC-Based Dimming”, filed May 29,2009, the entirety of the aforementioned applications being incorporatedherein by reference for all purposes.

BACKGROUND

Dimmers are commonly used in lighting and other applications to controlthe voltage and/or current level to a load such as a light. Manyconventional dimmers use thyristors such as a TRIAC or SCR to controlthe output voltage and/or current. Typically, the dimmer is designed toturn on only for a portion of each half cycle of an alternating current(AC) power supply. TRIACs may not turn on reliably with non-resistiveloads, so replacement of incandescent lights with more energy efficientlights such as light emitting diodes (LEDs) may not always be possibleor result in undesirable and/or unacceptable performance with atraditional TRIAC-based dimmer circuit in place. When the load currentis low, such as when using an LED light that, for example, uses aswitching power supply/driver, the TRIAC in the dimmer may not turn onwhen intended, or may flicker, flash or otherwise behave in undesirableways. The need to replace the dimmer when replacing incandescent lightswith LEDS can, depending on the situation, be expensive and/ordifficult.

Furthermore, thyristors may require a higher starting or trigger currentto initially start conducting than the holding current needed tocontinue conducting once started. In other words, the electrical currentthrough the thyristor may need to initially reach a higher startingcurrent level and can then be reduced to a level at or above the holdingcurrent. In practice, this means that a thyristor-based dimmer often hasto be turned up to a brighter level to turn on a light, before it can bedimmed down to the desired lower illumination. When a thyristor-basedlight dimmer is set to a dim level, fluctuations in the supply power canalso cause the current through the thyristor to drop below the minimumholding current and turn off the light.

SUMMARY

Various apparatuses and methods for controlling power to a load using athyristor are disclosed herein. For example, some embodiments provide anapparatus for controlling power to a load. The apparatus includes athyristor, a secondary load switchably connected to an output of thethyristor, and a sensor connected to the secondary load. The sensor isadapted to connect the secondary load to the thyristor when the outputof the thyristor falls below a predetermined level.

In an embodiment of the apparatus for controlling power to a load, theapparatus includes a lamp base, wherein the secondary load and sensorare located within the lamp base.

In an embodiment of the apparatus for controlling power to a load, thesecondary load is a resistor having a resistance below about 10 kilohms.

In an embodiment of the apparatus for controlling power to a load, thesecondary load is a motor, a fan, an incandescent light, an electricalcharger, or an air freshener.

In an embodiment of the apparatus for controlling power to a load, thesecondary load is switchably connected to the thyristor output by atransistor.

In an embodiment of the apparatus for controlling power to a load, thesensor includes a resistor connected between the thyristor output and acontrol input of the transistor, and a second resistor connected betweenthe control input of the transistor and a low voltage reference node,and a Zener diode connected to the control input of the transistor. Thesensor also includes a second transistor having an input connected tothe control input of the transistor, a third resistor connected betweenthe thyristor output and a control input of the second transistor, and afourth resistor connected between the control input of the secondtransistor and the low voltage reference node. The sensor also includesa second Zener diode connected to the control input of the secondtransistor.

In an embodiment of the apparatus for controlling power to a load, thesensor includes a current sensor connected to the thyristor output, anda comparator having a first input connected to an output of the currentsensor, and a second input connected to a reference signal, and anoutput connected to a control input of the transistor.

An embodiment of the apparatus for controlling power to a load has adelay element connected to the comparator.

In an embodiment of the apparatus for controlling power to a load, thesensor includes a voltage sensor connected to the thyristor output, acurrent sensor connected to the thyristor output, and a low voltage andlow current detector connected to the voltage and current sensors and toa control input of the transistor. The low voltage and low currentdetector is adapted to connect the secondary load to the thyristor whenthe thyristor output falls below a threshold voltage level or currentlevel.

An embodiment of the apparatus for controlling power to a load alsoincludes a filter connected to the low voltage and low current detector.

In an embodiment of the apparatus for controlling power to a load, thesensor includes a current monitor connected to the thyristor output andto a control input of the transistor. The current monitor is adapted toconnect the secondary load to the thyristor when the thyristor output iswithin a predetermined current range.

An embodiment of the apparatus for controlling power to a load alsoincludes a transformer having a first winding connected to the thyristoroutput, and a second winding connected to a primary load output. Thesensor includes a current sensor connected to the primary load output.The apparatus also includes a level shifter connected between thecurrent sensor and a control input of the transistor.

An embodiment of the apparatus for controlling power to a load alsoincludes a second current sensor connected to the first transformerwinding, with the second current sensor being connected to the controlinput of the transistor.

Some embodiments also provide a method of controlling power to a load,the method including monitoring an output of a thyristor-based dimmer,determining whether the output meets a predetermined condition, andconnecting a secondary load to the output while the output meets thepredetermined condition.

In an embodiment of the method, the determining includes identifyingwhen a current level through the output falls below a predeterminedlevel.

An embodiment of the method also includes isolating a primary loadoutput from the dimmer output through a transformer, wherein thedetermining comprises identifying when a current level through theprimary load output falls below a predetermined level. The method alsoincludes level shifting a low current indication signal.

In an embodiment of the method, the determining also includesidentifying when a current level through the dimmer output falls below apredetermined level.

In an embodiment of the method, the determining includes identifyingwhen a current level through the output and a voltage level at theoutput both fall below a predetermined level.

In an embodiment of the method, the determining includes identifyingwhen a current level through the output falls within a predeterminedrange.

An embodiment of the method also includes disconnecting the secondaryload from the output while the output does not meet the predeterminedcondition.

This summary provides only a general outline of some particularembodiments. Many other objects, features, advantages and otherembodiments will become more fully apparent from the following detaileddescription, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the various embodiments may be realized byreference to the figures which are described in remaining portions ofthe specification. In the figures, like reference numerals may be usedthroughout several drawings to refer to similar components.

FIG. 1 is a block diagram of a system using a thyristor startingcircuit.

FIG. 2 is a block diagram of a system using a thyristor startingcircuit.

FIG. 3 is a block diagram of a system using a thyristor startingcircuit.

FIG. 4 is a schematic illustration of a thyristor starting circuit usingan RC time constant delay with full-rectifier connection.

FIG. 5 is a schematic illustration of a thyristor starting circuit usingan RC time constant delay with half-rectifier connection.

FIG. 6 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit with sensing controller.

FIG. 7 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit with sensing controller.

FIG. 8 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit with sensing controller.

FIG. 9 illustrates a resistive load connected in parallel with an LEDpower supply, driver or other system.

FIG. 10 illustrates a switched resistive load switchably connected inparallel with an LED power supply, driver or other system.

FIG. 11 illustrates an LED power supply, driver or other system as itmay be connected through a transformer to a dimmer.

FIG. 12 illustrates a light socket adapter with a resistive load in thebase.

FIG. 13 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit.

FIG. 14 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit.

FIG. 15 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit with transformer-connectedcontrol.

FIG. 16 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit controlled via referencevoltage and with a transformer-connected load.

FIG. 17 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit controlled via referencevoltage and with a transformer-connected load.

FIG. 18 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit controlled via referencevoltage and time constant and with a transformer-connected load.

FIG. 19 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit controlled via referencevoltage and time constant and with a transformer-connected load.

FIG. 20 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit controlled via referencevoltage and time constant and with a transformer-connected load.

FIG. 21 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit activated on low voltage orcurrent and with a transformer-connected load.

FIG. 22 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit activated on high or lowcurrent and with a transformer-connected load.

FIG. 23 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit activated on high or lowcurrent and with a transformer-connected load.

FIG. 24 is a schematic illustration of a thyristor starting circuitand/or controlled thyristor load circuit activated on low input currentor low load current and with a transformer-connected load.

FIG. 25 is a flowchart of an example operation for starting a thyristor.

DESCRIPTION

The drawings and description, in general, disclose various apparatusesand methods for starting or igniting a thyristor, and variousapparatuses and methods for connecting a load across a thyristor-baseddimmer to ensure the thyristor operates properly with a primary loadthat is not purely resistive, both full time and when needed. Theapparatuses and methods disclosed herein may thus be adapted to apply aload to a thyristor at startup, or when a sensor detects that it isneeded or at all times, or in any combination of these. The term“thyristor” is used herein to refer to a bistable semiconductor device,either bidirectional or unidirectional, that can regulate the outputusing phase angle control or analogous control schemes, of which a TRIAC(triode for Alternating Current, or bidirectional triode thyristor) andan SCR (silicon-controlled rectifier) are particular non-limitingexamples. The term “starting circuit” is used herein to refer to acircuit that temporarily applies a load to the thyristor to help ensurethat the current through the thyristor reaches the minimum startingcurrent. The load is disconnected from the thyristor by the startingcircuit after a predetermined amount of time, or when a voltage orcurrent level in the system reaches a predetermined level, or based onany other suitable sensor measurement or control system determination.Note that the starting circuit may be applied when the thyristor-baseddimmer is first turned on, or at any time during operation of thethyristor-based dimmer as needed to enable low-current thyristor baseddimming. The circuits disclosed herein are also operational when nodimmer is in place, that is, they do not impede the normal functioningof typical loads or driver circuits if used without a dimmer.

Referring now to FIG. 1, a block diagram of a system using a thyristorstarting circuit is illustrated. A thyristor-based dimmer 10 isconnected to an AC (alternating current) input 12 such as a standard110V residential power outlet. The thyristor-based dimmer 10 is designedto reduce the voltage at the output 14, for example by chopping orturning off the output voltage for a portion of every cycle on the inputvoltage waveform. The thyristor-based dimmer 10 may include a knob,slider or other control allowing a user to select the desired outputlevel. In conventional applications, the output 14 of thethyristor-based dimmer 10 is connected directly to one or more lights16, enabling the user to select the desired illumination level of thelights. The thyristor-based dimmer 10 may also be connected to othertypes of loads 16 or to driver circuits for other types of lights, suchas LEDs or fluorescent lamps. However, as discussed above, the thyristorin the thyristor-based dimmer 10 may not operate properly with loads 16that are non-resistive, or with resistive loads when the dimmer isoperated at very low levels. The thyristor starting circuit 20 isconnected to the thyristor-based dimmer 10 to apply a resistive or othersuitable load to the thyristor in the thyristor-based dimmer 10 toenable proper operation. The thyristor starting circuit 20 may beconnected in series between the thyristor-based dimmer 10 and the load16, as illustrated in FIG. 1. In other embodiments, the thyristorstarting circuit 20 may be connected to the thyristor-based dimmer 10 inparallel with the load 16, as illustrated in FIG. 2. The thyristorstarting circuit 20 may also be integrated into a driver circuit 16 asillustrated in FIG. 3. Although FIGS. 1-3 show both lines of the ACinput 12 going through the thyristor-based dimmer 10, thethyristor-based dimmer 10 may be placed in series in one lead of the ACinput 12.

Referring now to FIG. 4, the thyristor starting circuit 20 may beconnected to the output 14 of the thyristor-based dimmer 10 through arectifier 22 to protect the thyristor starting circuit 20 from thenegative voltage half-cycles from the thyristor-based dimmer 10. Therectifier 22 may comprise any suitable device for rectifying the voltageat the output 14 of the thyristor-based dimmer 10, such as a diodebridge. Additional filtering components such as a capacitor (not shown)may be connected if desired across the rectifier 22.

A load 24 is switchably connected across the thyristor dimmer output 14through the rectifier 22. For example, a switch 26 such as a transistormay be connected in series with the load 24 across the thyristor dimmeroutput 14. The switch 26 may comprise an N-channelmetal-oxide-semiconductor field-effect transistor (MOSFET), a bipolarjunction transistor (BJT) or any other suitable switching device madefrom silicon (Si), gallium arsenide (GaAs), silicon carbide (SiC),gallium nitride (GaN) or any other suitable material. When the switch 26is closed the load 24 is connected to the thyristor-based dimmer 10, andwhen the switch 26 is open the load 24 is disconnected from thethyristor-based dimmer 10. The load 24 may comprise a simple resistor orany other suitable load to cause the thyristor to fire correctly. Theload 24 may also be a device that performs a useful function whenconnected, such as a small incandescent light, a heater, etc. The load24 is also referred to herein as a secondary load, to distinguish itfrom the primary load such as an LED or other type of light ornon-resistive light powered by the dimmer. In general, the lower theresistance of the secondary load 24, the more readily the thyristor willfire, as current through the thyristor is increased with the lowerresistance of the load 24. It is important to note that the term primaryload and secondary load is not related to the primary and secondarywindings of transformers used in some embodiments disclosed herein.Thus, for example, the term primary load is no indication of connectionto either a primary winding or secondary winding of a transformer alsoused in the circuit.

The transistor used as switch 26 should be rated to handle at least thepeak voltage that appears across the thyristor dimmer output 14. Thegate 30 of the switch 26 is powered, for example, by a pair of resistors32 and 34 connected between the supply node 52 and the return node 42.The gate 30 may also be protected by a Zener diode 36 or other voltagelimiting device or circuit, connected between the gate 30 and source 40,which is connected to the return node 42. The Zener diode 36 should beselected to keep the gate to source voltage on the switch 26 at or belowits maximum rated V_(GS).

The switch 26 is controlled by a second switch 44 connected between thegate 30 of the switch 26 and the return node 42. The second switch 44may also comprise an N-channel MOSFET transistor or any other suitabletype of switching device. When the switch 44 is off, the gate 30 of theswitch 26 is pulled up by resistors 32 and 34, turning on the switch 26and connecting the load 24 to the thyristor-based dimmer 10. When thesecond switch 44 is on, the gate 30 of the switch 26 is pulled downthrough the second switch 44, turning off the switch 26 anddisconnecting the load 24 from the thyristor-based dimmer 10.

The second switch 44 may be controlled by a number of variouscontrollers. In the embodiment of FIG. 4, an RC circuit is used tocontrol the second switch 44. A pair of resistors 46 and 50 is connectedbetween the supply node 52 and return node 42, with the common nodeconnected to the gate 54 of the second switch 44. The values of theresistors 46 and 50 are selected to safely turn on the second switch 44when the supply node 52 is at full power. Note that the second switch 44only needs a V_(DS) voltage rating equal to the maximum gate voltage ofthe switch 26 as set by the Zener diode 36. The gate 54 of the secondswitch 44 is protected by a Zener diode 56 or other voltage limiter,connected between the gate 54 and source 60 of the second switch 44. Acapacitor 62 is also connected between the gate 54 and source 60 of thesecond switch 44 so that the voltage at the gate 54 gradually rises to alevel set by the resistors 46 and 50 and the Zener diode 56 when poweris first applied to the system.

Values for the resistors 46 and 50 and the capacitor 62 are selected toprovide the desired RC time constant such that the load 24 is connectedto the thyristor-based dimmer 10 for the desired amount of time. Thisamount of time may be based on the characteristics of the particularthyristor in the thyristor-based dimmer 10, the characteristics of theload 16, the voltage level and stability of the AC input 12 and otherfactors. A relatively long or conservative amount of time may beselected to ensure that the thyristor will fire in all circumstances,although in ideal conditions the load 24 may be applied longer thanneeded, using a small amount of extra power. For example, if the timeconstant is adapted to connect the load 24 for a half second or even afew seconds, 60 to 240 half cycles will have reached the thyristor withthe load 24 applied. A shorter amount of time may be selected tominimize that power usage, at the risk of not firing the first time inparticularly poor environments. For example, the time constant may beadapted to connect the load 24 to the thyristor-based dimmer 10 for asmaller number of half cycles, such as 10 or 30. (Again, these timeconstants are merely examples and should not be viewed as limiting.)

The secondary load 24 may comprise a low value resistor such as a 500Ωor a 1kΩ resistor or higher or lower as needed or desired. The resistors(e.g., 32 and 34, 46 and 50) may comprise any circuit or device suitablefor providing reference voltages, such as resistive voltage dividers,capacitive or inductive voltage dividers, RC networks, bandgap referencevoltage circuits, etc. For example, in one embodiment, the resistors 32and 34, 46 and 50 have relatively high values to limit current drain onthe rectified input 52. The switches 26 and 44 may be any suitable typeof transistors, such as N-channel or P-channel field effect transistors,enhancement or depletion mode transistors, bipolar transistors, etc,made from any type of material including those based on silicon, galliumarsenide, gallium nitride, silicon carbide, silicon on insulator (SOI),silicon on sapphire (SOS), etc., with the logic of the thyristorstarting circuit 20 adapted accordingly to connect the secondary load 24across the thyristor dimmer output 14 when the rectified input at thesupply node 52 is at a low voltage level or low current level or in anyother state that would suggest that a thyristor in the thyristor-baseddimmer 10 may benefit from the application of the secondary load 24.

As illustrated in FIG. 5, the thyristor starting circuit 20 mayalternatively include a half bridge rectifier comprising two diodes 64and 66, both oriented in the same direction in the thyristor startingcircuit 20 so that the load 24 is applied to thyristor-based dimmer 10only in the positive half of every cycle at the AC input 12.

Referring now to FIG. 6, a sensor circuit 70 may be used to control theswitch 26 to connect and disconnect the load 24 with the thyristor-baseddimmer 10. The sensor circuit 70 may be powered for example by thethyristor dimmer output 14 via the supply node 52 and return node 42.The sensor circuit 70 has a control output 72 connected to thetransistor gate 30 to control the switch 26. A sense signal 74 is usedby the sensor circuit 70 to determine whether the load 24 should beconnected or disconnected. The sensor circuit 70 may be adapted for usewith any of a variety of sense signals 74. For example, the sense signal74 may be used to determine the voltage and/or current through thethyristor-based dimmer 10. In this case, the sensor circuit 70 isadapted to close the switch 26 and connect the load 24 when a lowvoltage and/or current is detected through the thyristor-based dimmer10. The sensor circuit 70 may be adapted in various non-limitingembodiments to use Boolean Algebra (i.e., NAND, NOR, AND, OR, ExclusiveOR, etc.) circuits, digital logic, including inverters, comparators,flip flops, counters, one shots, state machines, FPGAs,microcontrollers, microprocessors, etc., analog circuits such as opamps, current mirrors, time constants, integrators, voltage to frequencyand/or frequency to voltage converters, analog to digital converters,digital to analog converters, multipliers, log and antilog circuits,analog muxes, mixed signal circuits, etc. in any combination or form, inconjunction with one or more sensing inputs (e.g., 74).

In one embodiment, the sensor circuit 70 includes a microcontroller orother logic that only connects the load 24 if the dimmer 10 is active,for example by measuring the average voltage and allowing the load 24 tobe connected only if the average voltage is less than the averagevoltage expected on the AC line 12, or by measuring the duty cycle ofthe waveform at the thyristor dimmer output 14 to determine whether ithas been chopped by an active thyristor-based dimmer 10.

As illustrated in FIG. 7, the sense signal 74 may be scaled by a voltagedivider 80 and 82 if desired. For example, the resistors 80 and 82 maybe connected in series between the supply node 52 and return node 42. AZener diode 84 may be used to limit the voltage at the sense input ofthe sensor circuit 70. The rectified voltage across the supply node 52and return node 42 may be filtered by a capacitor 86. A time constantmay be applied to the sense signal 74 as illustrated in FIG. 8 by acapacitor 90 operating in conjunction with the resistors 80 and 82 toensure that the load 24 remains connected to the thyristor-based dimmer10 for at least some time duration.

Referring now to FIG. 9, a load 10 may also be connected across athyristor-based dimmer 10 full time to ensure that the thyristoroperates properly with a primary load 16 that is not purely resistive,such as a system of LEDs and/or LED driver circuits. The load 16 mayalso comprise any circuit suitable for powering an LED light from the ACinput 12. For example, given an AC voltage input 12 of about 110V thathas been phase chopped by the thyristor-based dimmer 10, the LED powersupply 16 may comprise a rectifier and current regulator to provide alow DC current as needed to power an LED light. The load 24 may comprisea small resistive load such as a resistor, an incandescent light, a fan,etc. The load 24 may alternatively be switchably connected across thethyristor dimmer output 14 in parallel with the load 16, as illustratedin FIG. 10 and described above. The switch 26 may be controlled by apulse signal or by a current or voltage sensing circuit, turning on whenthe current or voltage through the LED power supply or from the dimmeris at a low level.

Referring now to FIG. 11, the system may comprise a transformer 92between the thyristor-based dimmer 10 and the load 16, with thetransformer 92 applying enough of a resistive load to thethyristor-based dimmer 10 so that the thyristor in the dimmer 10functions properly.

The circuits described herein (e.g., 20), including in variousembodiments a resistive load, sensing and switching circuits, may beplaced in an adapter 94 for an Edison light socket or lamp base, such asan E26, E27, GU-10, MR16, PAR30, etc. as illustrated in FIG. 12. Theresistive load inside the adapter is connected in parallel with theload, and may have a fixed or variable resistance, and may be switchedor full-time.

The resistive load placed in parallel with the LED light or power supplymay be any small resistive load, applying for example a 4 watt load at115-125V, which would have a resistance of about 1-2 kohm. The smallresistive load may be a simple resistor dissipating electrical energythrough heat, or may be a device that performs some useful functionwhile providing enough of a resistive load for the thyristor in a dimmer10. For example, a small incandescent light such as a nightlight may beused for the resistive load in parallel with an LED lighting system. Theincandescent light may be shielded and hidden, or may be coverable bythe consumer or end user, or may be left visible to contribute to theoverall illumination. If left visible, the color of the incandescentlight may be adjusted using a colored filter adjacent the light. A glassor plastic window may partially or totally enclose the incandescentlight. Other examples of useful functions that may be performed whileadding a resistive load include a charger for an electrical storagedevice such as a battery charger or cell phone charger, a fan, anionizing air freshener, etc.

The system disclosed herein for adding a resistive load in parallel withan LED lighting system may comprise adding one resistive load 24 per LEDlighting system or one resistive load in parallel with multiple parallelloads. For example, given 200 1 W or 2 W LED lamps across a dimmer 10,only one parallel resistive load 24 is needed across the dimmer 10. Thisenables the user to include a single thyristor starting circuit 20 orparallel load 24 in a lamp base adapter 94 in one socket of achandelier, with LED drivers and/or lights connected to all of the othersockets. The single thyristor starting circuit 20 or parallel load 24enables the thyristor-based dimmer 10 operating the chandelier tofunction properly for all of the LED drivers and lights.

Referring now to FIG. 13, a load circuit 100 for low currentthyristor-based dimming may be sensor-based to apply the load 24 acrossthe thyristor dimmer output 14 whenever the supply node 52 falls below apredetermined threshold voltage. The AC output 14 of the dimmer isrectified in a rectifier 22. A transistor is used as a switch 26 toapply the load 24 across the thyristor dimmer output 14, either in adigital on/off fashion or in an analog fashion. The gate 30 of thetransistor 26 is powered by a pair 102 of resistors 32 and 34 based onthe rectified input 52, or by a network of capacitors, or any othersuitable voltage supply. The gate 30 of the transistor 26 may beprotected from high voltages from the pair 102 of resistors by a Zenerdiode 36 or other voltage-limiting device or circuit. The transistor 26is controlled by another switch or transistor 44, also biased by a pair104 of resistors 46 and 50 from the rectified input 52 and protected andassisted by a Zener diode 56. The time needed to turn on the switch 26and apply the load 24 can be controlled by a time constant, for exampleby including a capacitor 106 between the gate 30 and return node 42, inthis embodiment and in other embodiments discussed herein or invariations thereof. Time constants, filters or delays may be added inany other suitable manner as well, including analog methods and digitalmethods such as a digital delay.

The control transistor 44 is biased by the pair 104 of resistors 46 and50 such that it is off at low voltages on the rectified input 52 and onwhen the rectified input 52 reaches a predetermined voltage level andabove. The switching transistor 26 is biased by the pair 102 ofresistors 32 and 34 such that it would normally be on across theexpected operating voltage ranges for the rectified input 52. However,when the voltage at the dimmer output 14 and the rectified input 52 ishigh, the control transistor 44 is on, shorting the gate of theswitching transistor 26 down and turning off the switching transistor26. This isolates the secondary load 24 so that it does not appearacross the dimmer output 14. When the voltage at the dimmer output 14and the rectified input 52 is low, the control transistor 44 is off,allowing the gate of the switching transistor 26 to be biased up by thepair 102 of resistors 32 and 34 and turning on the switching transistor26. This connects the secondary load 24 across the dimmer output 14.Note that the resistors 32, 34, 46 and 50 are selected such that thesecond switch 44 is biased off and the switch 26 is biased on when thesupply node 52 is at a low voltage. This low voltage appears at thesupply node 52 when power is applied to the dimmer circuit but when thethyristor-based dimmer 10 potentially requires the secondary load 24 tooperate properly. The range of voltages in which the load 24 should beapplied may vary widely in different circumstances, depending on theline voltage, the design of the thyristor-based dimmer 10, various typesof loads, etc, and specific voltage ranges are therefore not givenherein. By switchably connecting the secondary load 24 across thethyristor-based dimmer 10 at low voltage levels, the dimmer 10 operatesreliably at low currents or loads.

Turning now to FIG. 14, in one embodiment the rectifier 22 may comprisea diode bridge or any other suitable circuit or device to rectify the ACoutput 14 of a dimmer 10. The secondary load 24 may comprise a low valueresistor such as a 500Ω or a 1 kΩ resistor or higher or lower as neededor desired. The resistors 32, 34, 46 and 50 and Zener diodes 36 and 56may comprise any circuit or device suitable for providing referencevoltages, such as resistive voltage dividers, capacitive or inductivevoltage dividers, RC networks, bandgap reference voltage circuits, etc.For example, in one embodiment, the resistors 32, 34, 46 and 50 compriseresistors of relatively high values to limit current drain on therectified input 52 through the resistors 32, 34, 46 and 50. Thetransistors 26 and 44 may be any suitable type of transistors, such asN-channel or P-channel field effect transistors, enhancement ordepletion mode transistors, bipolar transistors, etc, made from any typeof material including those based on silicon, gallium arsenide, galliumnitride, silicon carbide, silicon on insulator (SOI), silicon onsapphire (SOS), etc., with the logic of the circuit adapted accordinglyto connect the secondary load 24 across the dimmer output 14 when therectified input 52 is at a low voltage level or low current level or inany other state that would suggest that a thyristor such as a TRIAC orSCR in the dimmer 10 may benefit from the application of the secondaryload 24.

Turning now to FIG. 15, the transistor 26 switching the secondary load24 may be controlled by an operational amplifier (op-amp) or comparator110 rather than the control transistor 44 and resistors 46 and 50. Inthis embodiment, the current through the rectified input 52 is detectedusing a device such as a sense resistor 112. The sense resistor can belocated anywhere in the input current path with one location being belowthe primary switching transistor 120 as shown, for example, in FIG. 15although anywhere else in the input current path could also be used andsensed from the other chosen position(s). When the current is lower thana predetermined threshold established by a reference voltage 114, thecomparator 110 turns on the transistor 26 to connect the secondary load24. A resistor 116 may be included to pull down the gate of thetransistor 26 to ensure that it turns off when the output of thecomparator 110 is off. As discussed above, the secondary load 24 may beswitched based on a determination of low current through the dimmeroutput 14, low voltage, or any other suitable state indicator. The usein various non-limiting embodiments of Boolean Algebra (i.e., NAND, NOR,AND, OR, Exclusive OR, etc.) circuits, digital logic, includinginverters, comparators, flip flops, counters, one shots, state machines,FPGAs, microcontrollers, microprocessors, etc., analog circuits such asop amps, current mirrors, time constants, integrators, voltage tofrequency and/or frequency to voltage converters, analog to digitalconverters, digital to analog converters, multipliers, log and antilogcircuits, analog muxes, etc., mixed signal circuits, etc. in anycombination or form may be used in conjunction with one or more sensinginputs, nodes, etc. For example, an OR gate could be used to sense whenthe inputs detect either too low of a voltage or too low of a currentand respond appropriately. In another example, an AND gate can be usedto sense too low of a current and too low of a voltage. It should beunderstood that there are numerous and diverse ways to apply suchBoolean Algebra, digital logic, analog and digital circuits, FPGAs, etc.to the present invention and all of these numerous and diverseapproaches, methods, circuits, etc. are understood to be part of andincorporated into the present invention herein.

Although the circuit enabling low current or low voltage is not limitedto any particular primary load, load driving circuit, or load placement,one example is illustrated in FIG. 15. In this example, a switchingtransistor 120 turns current through a transformer 122 on and off.Current is supplied to a primary load 124 by the secondary winding ofthe transformer 122. A control circuit and driver for the transistor 120may also be connected between the transformer 122 and transistor 120.Portions of the control circuit and driver may be referenced to theprimary or secondary winding of the transformer 122 as desired, withisolation circuitry used between the secondary of the transformer 122and the transistor 120 to make the transition between primary-referencedcircuitry and secondary-referenced circuitry.

A more specific embodiment is illustrated in FIG. 16. In thisembodiment, the secondary of the transformer 122 provides power to theprimary load 124, e.g., an LED, with the voltage to the load 124conditioned as desired by elements such as capacitors 130 and diodes132. The current through the load 124 may be measured across a senseresistor 134, or any other suitable feedback information may be used asdesired, to control the transistor 120 providing power to the load 124through the transformer 122. The feedback from the sense resistor 134 orother suitable source may be level shifted or isolated in a levelshifting circuit 136, for example using an opto-isolator oropto-coupler. The level shifted feedback may be used by a pulsegenerator 140 or other control circuit to drive the gate of thetransistor 120. The transistor 120 may be driven in any suitable mannerto control the voltage and/or current to the load 124. As illustrated inFIG. 17, the driver or control circuitry used to control the transistor120 may be referenced to either or both the primary or secondary windingof the transformer 122 as desired. In other words, the driver or controlcircuitry may be located in full or in part on either side of the levelshifter 136. The driver or control circuitry may comprise any suitablecircuitry for controlling the transistor 120, for example, such as anoscillator of any appropriate type, a pulse width modulation circuit, apulse generator, a controlled pulse generator, or a microcontroller ormicroprocessor.

Turning now to FIG. 18, the timing used to control the transistor 26 maybe adapted using a time constant 142 applied using any suitablecircuitry in any appropriate location of the circuit. For example, an RCnetwork may be used at the input or output of the comparator 110 tocontrol the timing of the transistor 26 state changes.

The switchably connected secondary load 24 is not limited to use withany particular dimmer circuit. One embodiment is illustrated in FIG. 19,in which the voltage of an AC input 12 is varied by a TRIAC 146 or othertype of thyristor, controlled by dimmer control circuitry 150. Thesecondary load 24 is switchably connected in series with the TRIAC 146through a rectifier 22. As illustrated in FIG. 20, power conditioningcapacitors 152 and EMI filters 154 may be included as desired.

In other embodiments analog and/or digital control circuits 156 makeadditional determinations about the state of voltage, current or otherstates in the system to control the transistor 26. For example, asillustrated in FIG. 21, the transistor 26 is controlled by an OR gate160 connected to signals from the control circuits 156, a firstindicating that the current is too low and a second indicating that thevoltage is too low. If either condition is true, the OR gate 160 willturn on the transistor 26 and connect the secondary load 24. The inputsused by the control circuits 156 may be adapted as desired. For example,current conditions may be measured by the comparator 110 of previousembodiments or by a direct connection to the top of the sense resistor112 with appropriate amplification and signal conditioning if needed.Voltage conditions at the rectified input 52 may be measured, scaled andfiltered as desired by appropriate circuits 162.

As indicated above, the conditions used to switchably connect thesecondary load 24 and the logic or other control circuitry used todetect and act on those conditions may be adapted as desired. Forexample, as illustrated in FIG. 22, the transistor 26 may be turned onby an AND gate 166 when current conditions measured by the comparator110 are within a specified range, that is, not too high or too low asdetermined by comparator circuitry 170. As illustrated in FIG. 23, thismay be implemented by ensuring that the current level is greater than alower current setpoint (Setpoint1) and lower than an upper currentsetpoint (Setpoint2) as determined by comparator circuitry 172. Again,the current may be measured at any suitable point in the system andusing any suitable device or circuit element. Such circuitry may also beused in part or in the same fashion to modulate or turn off the primaryswitching transistor to the primary load so as to eliminate anyundesirable behavior, performance, etc. Also, the embodiments of theprimary and secondary circuit including whether the circuit containsinductors and/or transformers and the numbers of inductors and/ortransformers shown and contained within this document are meant to onlybe representative and not limiting in any way or form for the presentinvention. The present invention is not limited to any particularprimary circuit topology and can be generally applied to single andmultiple stage topologies including but not limited to buck, boost,buck-boost, boost-buck, CUK, forward, flyback, etc. and any combinationof these and other circuit topologies or from any load that is unable toprovide the proper load that the thyristor or related dimmer 10 orrelated circuit requires.

The circuitry used to detect and act on conditions in the system toswitchably connect the secondary load 24 may include microcontrollers ormicroprocessors, Boolean logic such as inverters, AND, OR, NAND, NOR,XOR or other logic elements, comparators and/or op-amps. Productcircuitry may be used, that is, for example, a multiplier which takesthe product of input voltage and input current to determine if inputpower is too low. The circuitry may also include amplifiers such asinstrumentation, summing, difference or error amplifiers. Log andantilog and log/antilog circuitry may be used. The circuitry may alsoinclude PID, adders, integrators and differentiators, etc. Hysteresismay also be incorporated in the circuitry used to detect and act onconditions in the system to switchably connect the secondary load 24.

Referring now to FIG. 24, the switch 26 may also be controlled based onmeasurements of the voltage and/or current levels through a load 124,optionally in conjunction with measurements of the voltage and/orcurrent levels at the rectified input 52. For example, in an embodimentwith a transformer-connected load 124 as in FIG. 24, the current throughthe load 124 is measured using the sense resistor 134, the result islevel-shifted by the level shifting circuit 136 such as an opto-isolatoror opto-coupler, and the result is used to turn on the switch 26 as wellas to control the pulse generator 140 and the current control transistor120. In other embodiments, separate level shifting circuits 136 may beprovided for these two functions. The voltage and/or current measurementthrough the load 124 may also optionally be combined with a voltageand/or current measurement at the rectified input 52 in an OR gate 160as illustrated in FIG. 24, or the switch 26 may be controlled solelybased on the load voltage and/or current by omitting the OR gate 160 andcomparator 110.

Note that the schematics illustrated herein are simplified to describethe functionality of the invention, and may omit various signalconditioning, level shifting, biasing and scaling components etc. Forexample, various transistors, AND gates, OR gates etc. are illustratedwithout the biasing components that may be needed for logic levels atthe inputs and outputs to be interpreted correctly. The specificadditional components needed are based on the type of logic used in thecircuits, the types of transistors and other components, etc, and arewell understood in the art and will not be described in more detailherein.

Referring now to FIG. 25, a method of starting a thyristor includesconnecting a load to the thyristor when power is first applied to thethyristor (block 200), leaving the load connected to the thyristor for apredetermined period (block 202), and disconnecting the load from thethyristor after the predetermined period (block 204).

The apparatuses and methods for starting or igniting a thyristor and forconnecting a load across a thyristor-based dimmer enable the use ofefficient lighting technologies such as LED lights to be used withexisting thyristor-based dimmers. The circuits may be embodied inlamp-base adaptors, in LED driver circuits or in any of a number ofother convenient locations. The circuits may also be used withtraditional incandescent lights or other loads to improve the behaviorof the thyristor-based dimmer at low settings with low output voltageand/or current.

While illustrative embodiments have been described in detail herein, itis to be understood that the concepts disclosed herein may be otherwisevariously embodied and employed. The configuration, arrangement and typeof components in the various embodiments set forth herein areillustrative embodiments only and should not be viewed as limiting or asencompassing all possible variations that may be performed by oneskilled in the art while remaining within the scope of the claimedinvention.

What is claimed is:
 1. A method of controlling power to a load, themethod comprising: monitoring an output of a thyristor-based dimmer;determining whether the output meets a predetermined condition; andconnecting a secondary load to the output while the output meets thepredetermined condition.
 2. The method of claim 1, wherein thedetermining comprises identifying when a current level through theoutput falls below a predetermined level.
 3. The method of claim 1,further comprising isolating a primary load output from the dimmeroutput through a transformer, wherein the determining comprisesidentifying when a current level through the primary load output fallsbelow a predetermined level, the method further comprising levelshifting a low current indication signal.
 4. The method of claim 3,wherein the determining further comprises identifying when a currentlevel through the dimmer output falls below a predetermined level. 5.The method of claim 1, wherein the determining comprises identifyingwhen a current level through the output and a voltage level at theoutput both fall below a predetermined level.
 6. The method of claim 1,wherein the determining comprises identifying when a current levelthrough the output falls within a predetermined range.
 7. The method ofclaim 1, further comprising disconnecting the secondary load from theoutput while the output does not meet the predetermined condition.